Filtering electric currents



Aug. 29, 1933. CROUSE 1,924,486

FILTERING ELECTRIC CURRENTS Original Filed Aug. l9 192 2 Sheets-Sheet l acy-l E 7 Z jlioad 61am wage.

1933- G. B. cRousE 1,924,486

FILTERING ELECTRIC CURRENTS Original Filed Aug. l9 192 2 Sheets-Sheet 2 Patented Aug. 29, 1933 UNITED STATES PATENT OFFICE George B. Crouse, New York, N. Y., assignor, by

mesne assignments, to Potter, Pierce 8; Schefflcr, Washington, D. Charles H. Potter, Richard L. Seheffler 0., a firm consisting of James F. Pierce, and

Application August 19, 1926. Serial No. 130,252 Renewed November '7, 1932 Claims.

admirably suited to the purpose, they are more particularly adapted to deliver moderate currents at relatively high voltages. In accordance with the present invention, power for heating the filaments of a plurality of audions in parallel may be economically obtained from the ordinary house supply as the filters are adapted to pass relatively high direct currents at a low voltage, while at the same time they are suitable for high voltages.

An object of the invention is to provide meth- .ods of and apparatus for filtering an alternating current ripple from a direct current supply which require smaller values of inductance and capacity than are possible with prior types of filters. An object is to provide a method of and apparatus for eliminating an alternating current ripple by passing the current through a number of circuit elements in series, the load being contained in one of said circuit elements, and magnetically coupling the series elements to increase the apparent inductance of the circuit containing the load. Further objects are to provide a filter which includes two series in'ductances so arranged that the varying voltage across one inductance establishes a neutralizing ripple current in the other inductance, and more particularly a filter of the type having as its elementary circuits, a source circuit, a'load circuit, and at least one additional circuit, in which the load circuit is coupled to at least two of the other circuits.

These and other objects which will be apparent from the following description may be attained with the embodiments of my invention which are illustrated in the accompanying drawings, in which:

Fig. 1 is a diagram representing a simple embodiment of the invention for explanatory purposes;

Fig. 2 is a simplified diagram of one form of the invention;

Fig. 3 is a simplified diagram of a modification;

is a schematic diagram showing the application of the invention to a radio set;

Fig. 5 shows a simplified diagram of a modification; and

Fig. 6 is a simplified diagram of a modification employing capacities.

Referring to Fig. 1, 1 represents a source of direct current upon which is superimposed an alternating current ripple by means of the alternator 2, the combined output of these two sources going to the wires 3 and 4. Connected between these wires are the two series circuit elements, one element represented by the impedance 5 and the inductance 6, the other series element comprising the impedance '7, the inductance 8, and the load circuit 9. The operation is then as follows:

The impedance 7 is preferably resistive or condensive reactance so that the bulk of the direct current is forced to flow through the load 9. The alternating component flows through the impedance 5 and sets up across the terminals of this impedance a certain voltage which is applied to the inductance 6. As this inductance is coupled to the inductance 8, it may be regarded as, a primary of a transformer of which 8 is the second ary. The alternating current after passing through impedance 5 passes through impedance 7 and sets up a voltage across its terminals which by proper adjustment of the impedance value may be made equal to the counter electromotive force generated in the inductance 8 by the inductance 6, It will be seen that the counter electromotive force in the circuit whose elements are '7, 8 and 9, is generated by transformer action from current flowing in the coil 6, and thus requires for its generation practically no alternating current to flow in the load circuit. There is thus secured a great increase in the apparent inductance in the load circuit.

While the above explanation is a general statement of the action of the invention, the individual current and voltage relations existing are so complex that they may best be stated mathematically by means of complex algebra.

In Fig. 2, E represents a source of alternating current potential, R1 a resistance, and L1 an inductance. In the case of a rectified alternating current source, R1 includes the mean resistance of the rectifiers, the transformer secondary resistance, and the resistance of L1. The inductance L1 may be only that due to the leakage of the transformer secondary, or may include an additional structure.

The resistances R2 and R3 correspond to the impedances 5 and 7 of Fig. 1. The inductances L2 and L3 correspond to the inductances 6 and 8 of Fig. 1. L20 and L30 represent the inductances due to leakage fiux in the physical structure of L2 and L2, or either or both may include additional inductance on separate structures. R20 represents the resistance of the windings L2 and L20. R30 includes the resistances of the windings L3 and Lao, and the load resistance.

The numerals I, II and III indicate the three elementary circuits which exist in the system and which must be considered in an analysis of the filter:

Circuit I includes the elements R1, L1, R2 and R3;

Circuit II includes the elements R2, R20, L20, L2;

and

Circuit III includes the elements R2, L3, L30, R30.

Designating the resistances of the circuits as inator, can the impedance become infinite. In circuits for filtering out the A. C. components from a rectified source, I have found that there is usually a strongly predominant frequency and it is highly desirable that the filter should have a higher impedance at this frequency than at any other.

This effect may be obtained by the addition of an additional circuit closed on itself, and coupled to both circuits II and III. This is shown diagrammatically in Fig. 3, the additional circuit shown at IV, comprising the inductance L4 and the resistance R4, and having the mutual inductances M2 and M3 between circuits IIIV and III-IV respectively. Also Zrv:jwL4 plus R4.

Analyzed by the method above, the impedance between the input and circuit III is, dropping numerically insignificant terms in the numerator,

Rx, R11 and Rm, respectively, the following values are apparent from an inspection of the circuits:

Equating the voltages in these three circuits separately, we have:

where 11, i2 and i3 are the instantaneous currents flowing in circuits I, II and III, respectively.

The solution of these equations for I3, the vector amplitude of the current flowing in circuit III, which includes the load, may be shown to be the fraction in the right hand member as the inversevalue of the effective impedance of the mesh between the input and output.

Therefore, writing the absolute value of this 1 impedance, and assuming that the inductances will be large in comparison with the resistances, we may write, as a practical approximation,

one and only one relation of R11 and R3, will make the denominator a minimum, and Z a maximum at a given frequency, and that under these conditions, the impedance will vary approximately directly as L1 and L3.

Further, Equation 5 shows that in no case, due

to the presence of the term R3 B11 in the denom- An examination of the denominator will show that, at a given frequency, the mesh may be so proportioned that both terms will simultaneously become zero, the numerator remaining finite, and therefore Z will at this frequency become infinite.

A novel feature should be pointed out in that, at a given frequency, an infinite impedance may be obtained, with resistance in all branches, and with no capacities in the network.

In practice, this circuit IV takes the form of a simple copper band linking the two cores 6 and 21 as shown at 8 Fig. 4.

In Fig. 4 I have shown the invention applied to a simple radio set in combination with the rectifier system shown in my patent numbered 1,764,791, granted March 12, 1929. In this figure, 30 and 31 represent the terminals of an alternating current supply feeding energy to a transformer having a primary 32 and an iron core 33. This transformer has two secondary systems, one comprising the coils 34 and 35 which are connected to the rectifier 36, and the impedance 37 as described in my Patent Number 1,704,791. The leads 16 and 19 thus form the output wires of a double-wave rectifier system for passing current to the filter and to the load, which comprises the audion filaments. A series coil 20 may be provided between the rectifier and the impedances 5, '7. The coil 6 is shunted across the resistance 5 and the circuit in parallel with the resistance 7 comprises the coil 8 in series with the load, and also the series coil 21. The inductances 6 and 8 are wound on the side members of a core 6 of the open-frame type, which core is preferably provided with an air-gap as the windings are traversed by the direct current as well as the alternating current ripple. It will be noted that the coil 8 also surrounds one side of a similar core 21 on which the coil 21 is Wound.

The B battery unit is supplied from another system of secondaries 38 and 39 connected to an impedance 40 and a rectifier 41. The filter comprises two series inductances 42, 43 and a shunt circuit connected across the line between the series inductance, which shunt circuit includes an inductance 44 and condenser 45 in series. Mutual inductance is provided between the three windings which are so designed that the filter presents a path in series with the load of high apparent inductance and a shunt path of low inductance. The apparent inductance of the coil 43 of the filter is further magnified by the series coil 46 which is located between the filter and the load. Terminals 47 and 48 of the filter system form the positive and negative terminals, respectively, for the plate current supply. The plate supply for amplifying tubes is taken directly from the positive terminal 47 and the supply for the detector tube or tubes is taken from an adjustable tap 49 on a potentiometer 50 which is connected across terminals 47 and 48. lhe negative terminal 48 is connected to one terminal of the filaments, and a condenser 51 is located between the detector tube plate circuit and the unfiltered filament current supply line to offset the effect of any residual ripple in the filament of this tube, as explained in my copending application, Ser. No. 733,558, filed August 22, 1924 (Patent 1,759,545, May 20, 1930).

I have found it of advantage in some cases to employ the modified mesh shown in Fig. 5, in which the parts identical with those in Fig. 2 are identically numbered. The impedance R3 of Fig. 2 is replaced in Fig. 5 by the complex impedance comprising R31 in parallel with the series elements R32 and L31.

The impedance of this mesh may be determined by replacing R3 in equation 4 by Z31, a quantity defined by the relation and then separating into real and imaginary 35 components as before.

Where it is required to filter alternating current components from a direct current source, and from considerations of economy or limita- 4 tions of the electrical output of the source, it is voltage which it is desired to filter out.

C1, C2 and C3 represent capacities, L2, L20, L3, L30 and M have been previously defined.

The resistances have been neglected, to effect simplification. In filters for high voltages at low .currents, where capacities would ordinarily be used, this will not greatly influence the result.

Circuit I includes E, C1, C2 and C3 Circuit II includes C2, L20 and L2 Circuit III includes C3, Lao and L3.

Let

then the differential equations of the three cir cuits are respectively:

And solving for 13, the vector current in the load circuit,

LML (0C2 wC from which Z1 may be written as before, by inverting the fractions and substituting the proper values of the individual impedances.

The four modifications of the invention shown above are merely typical examples and other combinations will suggest themselves to those skilled in the art, and they may be analyzed and computed by the methods given to determine the one best suited to the problem in hand.

It should also be pointed out that in every case the load and source may be interchanged in the mesh without altering the action of the system.

I claim:

1. An electrical filter for preventing the passage of a periodic ripple along a supply line delivering current to a direct current load, said filter comprising two inductances included in one side of said supply line and in series with each other and with said load, and means effective to induce a neutralizing ripple in one of said inductances by the periodic variation in the current traversing the other inductance; said means comprising impedance shunted across said supply line and having one terminal in common with the junction of said inductances, and an additional impedance constituting a series element in said supply line.

2. The invention as set forth in claim 1, wherein said additional impedance is shunted across that inductance which is not located at the load side of said shunt impedance.

3. The invention as set forth in claim 1, wherein the impedance of said shunt path is substantially free from capacitive reactance.

4. An electrical filter as set forth in claim 1 in combination with an additional circuit coupled to said inductances and effective to substantially completely neutralize a ripple of one frequency in the load circuit.

5. An electrical filter for eliminating a periodic ripple from current supplied to a direct current load, comprising two magnetically coupled inductances in series with the load, a shunt path for alternating current across one of said inductances, and a second alternating current path shunted across the second inductance and the load.

6. An electrical filter for eliminating an alternating current component from a current supply comprising two series inductances, an impedance shunted across one of said inductances, and a second impedance having one terminal connected to the junction of said inductances, the other terminal of said second impedance serving as a common terminal for the source of current and the load, the other terminals for the source and load being alternatively provided by the outer terminal of the shunt inductance-impedance and by the outer terminal of said second inductance.

7. An electrical filter for eliminating a periodic ripple from a direct current supply comprising two impedances in series and adapted to be connected across the supply, an inductance shunted across one of said impedances, and a second inductance adapted to be shunted across the other of said impedances through the load, said inductances being magnetically coupled to induce a neutralizing ripple current in said second inductance by the periodic variations in the potential drop across said first inductance.

8. In an electrical filter for selectively suppressing currents of a certain pre-assigned frequency, a plurality of intercoupled elementary circuits arranged in the line of current propagation, and another elementary circuit coupled to two of said plurality of circuits and located out of the direct line of current propagation.

9. In an electrical filter for completely suppressing alternating current of a certain preassigned frequency, a pair of series impedances having mutual impedance therebetween, a shunt impedance arm connected intermediate said series impedances, and a circuit non-resonant to the pre-assigned frequency coupled to the said series impedances and located outside of the direct line of curent propagation.

10. In an electrical filter, a pair of coupled inductances serially arranged in the line of current propagation, an impedance shunted across the line intermediate said inductances, a second impedance of the same type as said shunt impedance shunted across one of said inductances, and a circuit outside of the line of current propaga tion and coupled to said series inductances.

11. The invention as set forth in claim 10, in which said inductances are wound on a common core, and said circuit comprises a single band arranged on the said core.

12. In an electrical filter, two rectangular cores of the open frame type arranged with the side of one core adjacent one side of the other core,

a winding surrounding the said adjacent sides and an additional winding on each of said cores, all of said windings being serially connected in the line of current propagation, an impedance shunted across one of said additional windings, and a second impedance of like type shunted across the line at the junction of said first impedance and said winding which is common to the said cores.

13. The invention as set forth in claim 12, in combination with a closed band arranged in the magnetic field of both of said cores.

14. An electrical network comprising series and shunt impedances having the relative arrangement employed in a single section of the T-type, characterized by the fact that the series impedances comprising the respective cross arms are non-symmetrical in respect to the shunt impedance and cooperate with said shunt impedance to form at least three elementary circuits each of which is coupled to the other two, and impedance forming a fourth elementary circuit coupled to at least two of the said three elementary circuits.

15. An electrical network as set forth in claim 14, in which the impedance of each of the said four elementary circuits is positive, the impedances of said circuits and the couplings therebetween being so proportioned and arranged that the filter presents a substantially infinite impedance to waves of a finite frequency.

GEORGE B. CROUSE. 

